1. Field of the Invention
The present invention relates to a particulate positive electrode active material for a lithium secondary cell which favorably satisfies characteristics such as high charge and discharge cyclic durability, high safety, high temperature storage properties, a high discharge average voltage, large current discharge properties, a high weight capacity density and a high volume capacity density.
2. Discussion of Background
Recently, as the portability and cordless tendency of various electronic equipment have progressed, a demand for a non-aqueous electrolytic secondary cell which is small in size and light in weight and has a high energy density, has been increasingly high, and development of a non-aqueous electrolytic secondary cell having excellent characteristics has been desired more than ever before.
In general, a positive electrode active material employed for a non-aqueous electrolytic secondary cell comprises a composite oxide comprising lithium as the main active material and a transition metal represented by cobalt, nickel or manganese solid-solidified in lithium. Electrode characteristics such as the electric capacity, the reversibility, the operation voltage and the safety very depending upon the type of the transition metal to be used.
For example, non-aqueous electrolytic secondary cells employing an R-3m rhombohedral bedded-salt composite oxide having cobalt or nickel solid-solidified therein, such as LiCoO2 or LiNi0.8Co0.2O2, for the positive electrode active material, provide relatively high capacity densities of from 140 to 160 mAh/g and from 180 to 200 mAh/g, respectively, and provide favorable reversibility at a high voltage region of from 2.5 to 4.3 V. However, when such a cell is heated, the cell is likely to generate heat due to the reaction of the positive electrode active material and an electrolytic solvent at the time of charging.
JP-B-7-32017 discloses a positive electrode active material for a lithium secondary cell, having a part of Co in the composite oxide of LiCoO2 replaced with Ti, Nb or Ta to improve cyclic discharge properties. Further, Japanese Patent No. 2855877 discloses a positive electrode active material for a lithium secondary cell, having a part of Co in the composite oxide of LiCoO2 replaced with Zr to improve cyclic discharge properties. It is true that the cyclic discharge properties improve with these positive electrode active materials for a secondary cell, however, the safety tends to decrease, and the capacity tends to decrease also, such being unsatisfactory.
Further, Japanese Patent No. 3141858 discloses a positive electrode active material for a lithium secondary cell comprising a composite oxide of LiaMbOcXd (wherein M is at least one member of Cr, Mn, Fe and Co, and X is a halogen such as fluorine). A positive electrode of such a composite oxide is considered to provide favorable characteristics such as cyclic properties and storage stability even at a temperature of at least 50° C. However, of the positive electrode comprising such a positive electrode active material, although the cyclic durability improves or the storage stability improves, on the other hand, the initial capacity density and the discharge average voltage tend to decrease, and the large current discharge properties are unsatisfactory, and further improvement has been desired.
As described above, among conventional positive electrode active material for a lithium secondary cell, no positive electrode active material which satisfies characteristics such as high safety, high charge and discharge cyclic durability, high temperature storage properties, a high discharge average voltage, large current discharge properties, a high weight capacity density and a high volume capacity density in a well-balanced manner, has been obtained.
It is an object of the present invention to provide a particulate positive electrode active material for a lithium secondary cell, which favorably satisfies characteristics such as high safety, high charge and discharge cyclic durability, high storage properties, a high discharge average voltage, large current discharge properties, a high weight capacity density and a high volume capacity density in a well-balanced manner.